Markus and Thomas,
Yes, I'm aware of http://ontoworld.org/wiki/Semantic_MediaWiki (I believe I referenced it in my original post + cc'd the semediawiki list(s)?).
I call my project 'semarbtri' to distinguish it from the existing Semantic Mediawiki project.
My biggest beef w/ Semantic Mediawiki (the reason I'm writing my own extension) is that you can only add semantic information for the page you're editing.
For example, on the Lucy page, you could say:
Lucy has a younger brother [[has_brother::Linus]]
However, you couldn't add any semantic information about Linus on the Lucy page. Instead, you'd have to create a new page for Linus, and add the semantic information there.
This can be really tedious if you're listing all the characters in a comic strip (for example)-- you have to create pages for each one, instead of just including the semantic info when you list them.
In my opinion, this is why startup wikis end up w/ so many blank pages.
I think having a plugin that lets you add info to other pages would be really helpful.
That's an interesting point. Semantically, a different person should be on a different page, but you make a good point that in practice, that isn't always the best option. I would suggest adding your feature to the existing extension, rather than reinventing the wheel, though.